Various contrast enhancement methods have been proposed as techniques for generating a sharp image by correcting a low-contrast image.
The gradation correction device described in Patent Document 1, for example, generates a luminance signal from primary color signals, detects information about the distribution of the generated luminance signal over the entire screen, creates a gradation correction conversion table for smoothing the histogram distribution over the entire range of luminance levels, calculates correction coefficients based on the input/output ratio of the gradation correction conversion table, and multiples each primary color signal by the same correction coefficients, thereby raising the contrast of the entire image.
The image processor described in Patent Document 2 generates smoothed image data in which edge components of the input image data are preserved, then amplifies the differences between the input image data and smoothed image data and adds them to the smoothed image data to enhance the contrast of high-frequency components of the image.
The general aim of the above contrast enhancement processes is to widen the range of the luminance distribution in the image as a whole or in local areas. A problem with stretching the range of the luminance distribution, however, has been that it amplifies not only the signal but also noise in the stretched luminance levels, impairing the quality of the image.
After gradation correction of an image, the imaging apparatus in Patent Document 3 accordingly separates the image into a plurality of frequency components by means of a wavelet transformation and reduces noise by coring the plurality of frequency components, setting the coring thresholds on the basis of the gradation correction curve so that noise components are suppressed according to the degree to which noise is amplified during the gradation correction.